Effect of Higher-Order Hydrocarbons on the Ignition of Lean Methane-Air Mixtures at Elevated Pressures

摘要

Because of their importance as likely fuel sources for power generation and other applications, fuel blends composed of methane and higher-order hydrocarbons are of interest. Of particular importance are relevant ignition chemistry data at elevated pressures and temperatures typical of engine applications. Shock-tube ignition experiments at elevated temperatures are commonly used to provide such information, but few data exist on CH4 blends other than those typical of natural gas, and few data were available until only recently at pressures in the 10-to-20-atm range. Previous lower-pressure shock-tube data on methane-alkane blends can be found in Crossley et al.[1], Eubank et al. [2], and Zellner et al. [3]. Experiments using methane-propane blends were performed by Frenklach and Bornside [4], and methane-butane experiments can be found in Higgin and Williams. Of interest herein are lean methane-hydrocarbon blends with appreciable levels of higher-order hydrocarbons, with the hydrocarbon content being as much as 20-30% or more of the gaseous fuel blend by volume. In a recent study [6], the authors performed a series of shock-tube experiments on methane blends concentrating primarily on CH_4/H_2 and CH_4/C_2H_6 in lean fuel-air mixtures (φ = 0.5) at pressures up to 25 atm. The significant result of these experiments is the dramatic accelerating effect the H_2 and C2H6 have on methane ignition. For example, a 30% addition of ethane reduced the ignition delay time relative to a pure methane fuel by more than a factor of ten over a range of temperatures from 1100-1500 K. A chemical kinetics model was put forth to model the higher-pressure methane-only mixtures and was extended to include the CH_4/H_2 data by Hall and Petersen. Presented in this paper are some results from a follow-on experimental study of fuel-lean mixtures in air for methane-hydrocarbon blends with the hydrocarbon being propane, butane, or pentane. Some details on the experiment are covered first, followed by typical ignition delay time results.